Optical waveguides, which are a promising transmission medium for optical communication systems, normally consist of an optical fiber having a core of transparent material having a refractive index n.sub.1 surrounded by a layer of transparent cladding material having a refractive index n.sub.2 which is lower than n.sub.1. A plurality of waveguide fibers are often disposed in side-by-side relation to form a fiber bundle in order to propagate to the receiver more light than can be carried by a single fiber and to provide redundancy in the event that some fibers break.
It has been known for some time that light can be propagated along a transparent fiber structure having a refractive index that is higher than its surroundings, and clad fibers have been employed to transmit light over relatively short distances. To introduce light into such a fiber from a medium having a refractive index n.sub.o, the light must be directed toward the fiber endface within a meridional entrance cone having a half angle .theta..sub.c measured from the fiber axis, wherein ##EQU1## The numerical aperture (N.A.) of such a fiber, which is a measure of the light gathering ability thereof, is defined as follows: EQU N.A. = n.sub.o sin .theta..sub.c = (n.sub.1.sup.2 - n.sub.2.sup.2).sup.1/2 (2)
In conventional optical fibers the difference between the refractive indices of the core and cladding material is made quite large so that the NA is large, and therefore the fiber is capable of gathering a relatively large amount of light emitted by a source. However, the difference between the refractive indices of the core and cladding of optical waveguides is maintained small to avoid severe distortion of the signal envelope and because presently available materials capable of providing large index differences tend to be lossy.
In accordance with equation (2) this small difference between core and cladding refractive indices in optical waveguides results in a numerical aperture that is smaller than that of conventional optical fibers. Whereas the numerical aperture of commercial optical fibers or light pipes of the conventional type may be as high as about 0.6, the numerical aperture of an optical waveguide is usually between about 0.10 and 0.15, corresponding to an entrance half angle of about 5.degree. to 10.degree..
Due to the relatively low acceptance angles or numerical apertures exhibited by optical waveguides, radiation from the waveguide light source must be highly directional to efficiently couple to the waveguide or waveguide bundle. Since a coherent light source having the described characteristics can yield efficient coupling, lasers have usually been considered for this role. However, it is often desirable to utilize incoherent light sources such as light emitting diodes, lamps and the like as light sources for optical waveguides. Solid state sources, for example, are advantageous in that they are more rugged and compact than conventional lasers and are more compatible with solid state circuitry. However, the coupling of optical energy directly from such a source to an optical waveguide bundle is extremely inefficient due to the spatial and angular distribution of the source output energy and the low numerical aperture of the fiber bundle. For example, when the light emission distribution is Lambertian, which is a good approximation of the light emission from light emitting diodes, less than two percent of the total emitted radiation falls within the entrance angle exhibited by present low loss optical waveguides. This represents a severe loss in optical coupling when the surface of a diode is disposed in contact with an optical waveguide bundle.
Some light emitting diodes are provided with lensshaped transparent covers over the emitting area. Even though this causes the emitted light to be more directive, the coupling loss between such a diode and an optical waveguide bundle is so great as to preclude the use of such diodes without the use of additional light collimating means. For example, the coupling loss for a Monsanto MV10A diode having a transparent dome disposed adjacent to the end of a waveguide fiber bundle was measured to be 26dB. Losses of this magnitude cannot be tolerated in optical communication systems, especially in view of the fact that waveguide attenuation is as low as about 4dB/km.
In an attempt to decrease coupling losses the lens-shaped housing was removed from the light emitting diode by grinding and polishing the housing close to the light-emitting surface thereof, and a lens system was disposed between the diode and the bundle endface. The minimum loss obtained by such an arrangement was 12.3dB for an expensive microscope objective lens system which was relatively difficult to align.